1. Field of Invention
The present invention relates to electrochemical gas sensors, and more particularly, to electrochemical gas sensors for measuring acid gases.
2. Description of Related Art
Electrochemical sensors are widely used for measuring concentration of toxic gases. It is particularly important for an electrochemical sensor, used for gas detection, to possess high sensitivity, high accuracy, low (fast) response time and stability of characteristics in time. It is also essential that the sensor is selective, e.g., gases other than the one to be detected should cause small perturbations or preferably no signal at all.
In general, to be useful as an electrochemical sensor, a measuring and counter electrode combination must be capable of producing an electrical signal that is related to the concentration of the analyte. In addition to a measuring and counter electrode, an electrochemical sensor often includes a third electrode, commonly referred to as a reference electrode. A reference electrode, is used to maintain the working electrode at a known voltage or potential.
Generally, the electrodes of an electrochemical cell provide a surface at which an oxidation or a reduction reaction occurs involving the analyte. The electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte gas (that is, the gas to be detected) at the measuring electrode. The measurable current arising from the cell reaction is directly proportional to the rate of reaction.
When measuring an electrochemically active gas, the gas makes contact with the measuring electrode and a process of oxidation or reduction of the gas molecules takes place, according to the type and character of the gas and the defined potential of the measuring electrode. Therefore, the measuring electrode acts as a catalyst for a RedOx process. Such a system is used for example to measure electrochemically active hydrogen sulfide. Hydrogen sulfide, upon contact with a catalytic platinum measuring electrode, is oxidized directly to sulfuric acid. Thus, the measuring electrode acts only as a catalyst for the oxidation process.
When the target gas is not electrochemically active, i.e., it cannot be oxidized or reduced, then other types of gas sensors must be employed. Suitable sensors for such non-electrochemically active gases include sensors that operate based on the principle of a precursor chemical reaction involving the participation of a target gas in the area of a three-phase boundary of the measuring electrode. The three-phase boundary is formed by the solid phase of the electrochemically active catalyst (electrocatalyst), the liquid phase of the electrolyte and the gas phase of the measuring gas. The product of the precursor chemical reaction is then electrochemically oxidized or reduced on the measuring electrode. This precursor chemical reaction principle has been applied using hydrogen fluoride (HF), a target gas that is not electrochemically active.
One of the known HF sensors, described in U.S. Pat. No. 3,795,58, incorporates the principle of a precursor chemical reaction involving the participation of hydrogen fluoride gas. Sensors of such type include measuring, reference and counter electrodes in contact with a water-based electrolyte. The electrolyte contains compounds that can be oxidized and/or reduced such as bromate/bromine-salts. Specifically, the acid properties of hydrogen fluoride are used to initiate the chemical reaction. As HF is adsorbed by an electrolyte, it forms ions H3O+(Reaction 1), which take part in the following reaction (Reaction 2).
HF+H2O=H3O++F-xe2x80x83xe2x80x83(1)
5Br- +BrO3- +6H+=3Br2 +3 H2Oxe2x80x83xe2x80x83(2)
A pH shift in the electrolyte near the measuring electrode occurs in the presence of HF (Reaction 1). The second reaction (Reaction 2) proceeds with considerable rate only if the pH is close to or lower than 4. The generated bromine in the second reaction, cooperating with the electrocatalyst of the measuring electrode, is then reduced to a bromide ion as shown (Reaction 3).
Br2+2e=2Br-xe2x80x83xe2x80x83(3)
The reduction current is a measure of the concentration of the HF in the gas. Thus the electrochemical reaction is an indirect process where a precursor chemical (Reaction 2) takes place before an electrochemical reaction occurs.
The drawback of this type of HF sensor is an internal effect ,which leads to a sensitivity decrease and an increase of response time during the life time of the sensor. On storing or during operation for several months, the electrolyte forms a small concentration of bromine due to a high amount of bromide ions already in the bulk electrolyte of the sensor. This in turn leads to a shift of the potential of the reference electrode as well as of the measuring electrode and thus to a lower sensitivity and slower response time in the presence of HF.
An additional reason for the slower response time is a shift of the pH in the electrolyte to higher values due to internal corrosion processes at the reference and counter electrodes. This is due to the high amount of bromate ions of the electrolyte. This bulk electrolyte pH increase leads to a delay in pH shift to the lower pH level necessary for starting reaction (Reaction 2) in the vicinity of the working electrode on exposure to HF gases.
It will be appreciated from the foregoing that there is a need in the acidic gas sensing field for an electrochemical sensor that overcomes the aforementioned problems of the prior art.
In particular, what is needed is a sensor that responds relatively quickly to changes of pH in the electrolyte, and eliminates the formation of unwanted by-products in the electrolyte that decrease sensitivity.
It is desirable, therefore, to develop an improved electrochemical gas sensor and electrodes for the detection of non-electrochemically active gases, which mitigate or substantially eliminate one or more of the above drawbacks.
In accordance with the present invention an improved electrochemical sensor is provided for measuring of non-electrochemically active gases. The improved electrochemical sensor has improved accuracy within its designed operating range, improved response speed, and remains stabilized over its designed operating lifetime.
Briefly and in general terms, the present invention provides an electrochemical cell and method for measuring the concentration of non-electrochemically active gases that form protons upon disassociation in an aqueous electrolyte, such as hydrogen fluoride (HF). On a fundamental level, the non-electrochemically active gaseous molecules are adsorbed on an electrolyte, dissociated into ions; these ions participate in a chemical reduction of reducible compounds located on a measuring electrode. This electrochemical cell advantageously comprises structural components and compounds that reduce the introduction of protons from sources other than the non-electrochemically active gases.
The electrochemical cell in one embodiment comprises a cell body including an electrolyte chamber for an aqueous electrolyte. The electrolyte chamber has a plurality of electrodes therein. These electrodes include an electrochemically active measuring electrode, a counter electrode and a reference electrode, with the respective electrodes being spaced apart from each other and mounted to the electrochemical cell body. The aqueous electrolyte electrically communicates the three electrodes by contacting all of them. An electrical circuit, connected to the electrodes, is constructed and arranged to quantify the current generated by the chemical reaction within the electrochemical cell.
The surface of the electrochemically active measuring electrode comprises a layer of an electrochemically active compound that is reducible in the presence of decreased pH levels in the aqueous electrolyte.
Preferably, the compound is a reducible metal oxide, and more preferably, a reducible metal oxide having a positive reduction potential that limits generation or consumption of protons from sources other than the proton-forming, non-electrochemically active gases. Most preferably, the reducible metal oxide is manganese dioxide or lead dioxide.
The surface of the counter electrode comprises a layer of an electrochemically active compound that is oxidizable, and preferably without the introduction of protons into the electrolyte. The aqueous electrolyte advantageously comprises a hygroscopic compound that is effective in preventing the evaporation of water from the electrolyte sensor, and preferably, the selected hygroscopic compound does not form protons in solution or in a pH buffer system.
The present invention in another aspect provides a method of using an electrochemical gas sensor comprising a measuring electrode, a counter electrode, a reference electrode and an aqueous electrolyte communicatively connecting the electrodes. In such sensor, the measuring electrode has an electrochemically active surface comprising a stable reducible metal oxide. The stable reducible metal oxide is reduced when the pH of the electrolyte is lowered due to the presence of increased proton concentration.
Such method comprises the steps of:
a) placing the electrochemical gas sensor in communicative connection with an environment containing a proton forming non-electrochemically active acid gas that is suitable to increase the proton concentration in the aqueous electrolyte to cause a chemical reduction of the reducible metal oxide on the measuring electrode; and
b) measuring the current flow between the measuring electrode and the counter electrode to obtain a measurement of the concentration of non-electrochemically active acid gas in the environment.
Other aspects, features and advantages of the invention will become more fully apparent from the ensuing description, drawings and appended claims.